1. Field of the Invention
The invention relates to a hybrid vehicle and a method for controlling a hybrid vehicle.
2. Description of the Related Art
Japanese Patent Application Publication No. 11-280457 (JP-A-11-280457) describes a technology for a vehicle that uses only an internal combustion engine as a drive power source for the vehicle. According to the technology, the amount of oxygen stored in a three-way catalyst during fuel supply cutoff period is calculated. When the fuel-supply cutoff is cancelled to restart fuel supply, the air-fuel ratio is made richer by an amount based on the calculated amount of oxygen stored in the three-way catalyst. Japanese Patent Application Publication No. 2000-161098 (JP-A-2000-161098) describes a technology for a vehicle that uses a lean burn engine as a drive power source and that includes an automatic transmission. According to the technology, when the shift mode of the automatic transmission is switched from the normal mode, where a higher priority is given to fuel efficiency, to the power mode, where a higher priority is given to accelerating performance, the target air-fuel ratio map for the lean burn engine is switched from the target air-fuel ratio map for normal mode to the target air-fuel ratio map for power mode. The lean air-fuel ratio range for the power mode is narrower than that for the normal mode. Japanese Examined Patent Application Publication No. 06-6929 describes a technology for a vehicle that uses an internal combustion engine as a drive power source for the vehicle. According to the technology, an oxygen concentration sensor that outputs a signal indicating a value proportional to the oxygen concentration in the exhaust gas is provided, and a target air-fuel ratio is set based on a predetermined engine operation parameter, and the target air-fuel ratio is corrected based on the selected gear of the transmission. Then, the air-fuel ratio of the air-fuel mixture to be supplied to the internal combustion engine is controlled, in a feedback manner, to the corrected target air-fuel ratio based on the value indicated by the signal transmitted from the oxygen concentration sensor.
Hybrid vehicles, in which an internal combustion engine and at least one motor generator are used in combination to produce power for the vehicle, have recently become widespread. Also, applying a shifting device, which selects a desired operation mode from among multiple operation modes in response to a change in the shift position, to a hybrid vehicle has been proposed to satisfy various needs of drivers. In this case, the multiple operation modes define the respective different ranges, in which the required drive power required to cause the vehicle to run can be set, and the respective different operation point constraints, under which the target speed of the internal combustion engine corresponding to the required drive power is set. Such shifting device is applied to the hybrid vehicle. When an instruction to decelerate the vehicle is issued, for example, when an accelerator pedal is released, the speed of the internal combustion engine, in which fuel supply is cut off, is forcibly changed by the motor generator, whereby a braking force corresponding to the selected shift position is generated by applying engine braking. However, when a driver is allowed to select any one of the shift positions (operation modes), fuel-supply may be frequently cut off in response to instructions for deceleration. If fuel-supply is frequently cut off, a great amount of air is supplied to an exhaust gas purification catalyst, which causes adhesion of oxygen to the catalyst. As a result, the NOx purification efficiency of the catalyst may be reduced.
The invention provides a hybrid vehicle in which reduction of the NOx purification efficiency of an exhaust gas purification catalyst is suppressed when selection of one of the operation modes which define respective different ranges, where a required drive power required to cause the vehicle to run can be set, is allowed, and a method for controlling such hybrid vehicle. The invention also provides a hybrid vehicle in which reduction of the NOx purification efficiency of an exhaust gas purification catalyst is suppressed to purify the exhaust gas more effectively, and a method for controlling such hybrid vehicle.
The following configurations are employed in a hybrid vehicle and a method for controlling a hybrid vehicle according to the invention.
A first aspect of the invention relates to a hybrid vehicle including an internal combustion engine; a purifying device including a catalyst that purifies exhaust gas discharged from the internal combustion engine; an electric power storage device; an electric power/power reception/output device that is connected to a first axle and an output shaft of the internal combustion engine, and that receives power from the output shaft and outputs the power to the first axle or receives power from the first axle and outputs the power to the output shaft while exchanging electric power with the electric power storage device and exchanging power with the internal combustion engine; an electric motor that receives power from the first axle or a second axle that differs from the first axle or outputs power to the first axle or the second axle, and that exchanges electric power with the electric power storage device; an operation mode setting device that sets an execution operation mode to one of multiple operation modes which define respective different ranges where a required drive power required to cause the vehicle to run can be set, and that allows, when the execution operation mode is set to a predetermined operation mode that is one of the multiple operation modes, a driver to select any one of operation modes included in the predetermined operation mode; a target air-fuel ratio setting device that sets a target air-fuel ratio for the internal combustion engine under a first constraint when the driver is not allowed to select any one of the operation modes included in the predetermined operation mode, and that sets the target air-fuel ratio under a second constraint, under which the target air-fuel ratio tends to be set to a value richer than the target air-fuel ratio set under the first constraint, when the driver is allowed to select any one of the operation modes included in the predetermined operation mode; a required drive power setting device that sets the required drive power according to the execution operation mode set by the operation mode setting device; and a controller that controls the internal combustion engine, the electric power/power reception/output device, and the electric motor so that an air-fuel ratio in the internal combustion engine becomes equal to the target air-fuel ratio set by the target air-fuel ratio setting device and a drive power corresponding to the required drive power set by the required drive power setting device is output.
In the hybrid vehicle according to the first aspect of the invention, the execution operation mode is set to one of the multiple operation modes which define respective different ranges where a required drive power required to cause the vehicle to run. The multiple operation modes include the predetermined operation mode at which the driver is allowed to select any one of the operation modes included in the predetermined operation mode. When the driver is not allowed to select any one of the operation modes included in the predetermined operation mode, the target air-fuel ratio for the internal combustion engine is set under the first constraint. On the other hand, when the driver is allowed to select any one of the operation modes included in the predetermined operation mode, the target air-fuel ratio is set under the second constraint. Under the second constraint, the target air-fuel ratio is set to a value richer than the target air-fuel ratio set under the first constraint. The internal combustion engine, the electric power/power reception/output device, and the electric motor are controlled so that the air-fuel ratio in the internal combustion engine becomes equal to the set target air-fuel ratio, and the drive power corresponding to the required drive power set according to the set execution operation mode is output. Namely, when the driver is allowed to select any one of the operation modes, it is assumed that fuel-supply cutoff in response to an instruction to decelerate the vehicle will be performed relatively frequently. If no measures are taken in such a case, the NOx purification efficiency may be reduced, because a great amount of air is supplied to the exhaust gas purification catalyst due to fuel-supply cutoff and, therefore, oxygen adheres to the exhaust gas purification catalyst. To avoid such inconvenience, when the driver is allowed to select any one of the operation modes included in the predetermined operation mode, the target air-fuel ratio is set to a value richer than the target air-fuel ratio set when the driver is not allowed to select any one of the operation modes included in the predetermined operation mode. Thus, the amount of oxygen supplied to the purifying device can be reduced before fuel-supply is cut off or when fuel-supply is restarted after fuel-supply is cut off. As a result, even if a relatively great amount of air is supplied to the purifying device when fuel-supply is cut off, reduction in the NOx purification efficiency of the catalyst is suppressed, and, consequently, exhaust gas is purified more effectively.
The hybrid vehicle according to the first aspect of the invention may further include a catalyst warm-up determination device that determines whether warm-up of the purifying device has been completed. The target air-fuel ratio setting device may set the target air-fuel ratio under the second constraint, when the driver is allowed to select any one of the operation modes included in the predetermined operation mode and it is determined that warm-up of the purifying device has not been completed. Generally, if a great amount of air is supplied to the exhaust gas purification catalyst due to fuel-supply cutoff while the purifying device has not been warmed sufficiently, the NOx purification efficiency of the catalyst may be reduced significantly. Accordingly, when the driver is allowed to select any one of the operation modes and it is determined that warm-up of the purifying device has not been completed, the target air-fuel ratio is set to a richer value. As a result, it is possible to suppress reduction in both fuel efficiency and NOx purification efficiency of the catalyst.
In the first aspect of the invention, the operation mode setting device may be a shift position setting device that sets an execution shift position to one of multiple shift positions in response to an shift operation performed by the driver, the multiple operation modes may correspond to the respective multiple shift positions, and the multiple shift positions may include a sequential shift position at which the driver is allowed to select any one of shift positions included in the sequential shift position.
In the first aspect of the invention, the operation modes that the driver is allowed to select when the sequential shift position is selected may define respective different ranges, in which the required drive power can be set, and respective different operation point constraints, under which a target speed of the internal combustion engine corresponding to the required drive power is set. The controller may control the internal combustion engine, the electric power/power reception/output device, and the electric motor so that the internal combustion engine is operated at the target speed set under the operation point constraint and the drive power corresponding to the required drive power is output, when an instruction to decelerate the vehicle is issued and fuel-supply to the internal combustion engine is cut off while the driver is allowed to select any one of the operation modes included in the predetermined operation mode. Thus, the driver's demand is satisfied by changing the braking force generated by engine braking for each operation mode.
In the first aspect of the invention, the electric power/power reception/output device may include a three-axis power reception/output device that is connected to three shafts that are the first axle, the output shaft of the internal combustion engine, and a third shaft, and that receives or outputs, based on power received from and/or output to any two of the three shafts, power from or to the remaining shaft; and an electric motor that receives power from or outputs power to the third shaft.
A second aspect of the invention relates to a method for controlling a hybrid vehicle including an internal combustion engine; a purifying device including a catalyst that purifies exhaust gas discharged from the internal combustion engine; an electric power storage device; an electric power/power reception/output device that is connected to a first axle and an output shaft of the internal combustion engine, and that receives power from the output shaft and outputs the power to the first axle or receives power from the first axle and outputs the power to the output shaft while exchanging electric power with the electric power storage device and exchanging power with the internal combustion engine; an electric motor that receives power from the first axle or a second axle that differs from the first axle or outputs power to the first axle or the second axle, and that exchanges electric power with the electric power storage device; and an operation mode setting device that sets an execution operation mode to one of multiple operation modes which define respective different ranges where a required drive power required to cause the vehicle to run can be set, and that allows, when the execution operation mode is set to a predetermined operation mode that is one of the multiple operation modes, a driver to select any one of operation modes included in the predetermined operation mode. According to the method, a target air fuel ratio for the internal combustion engine is set under a first constraint when the driver is not allowed to select any one of the operation modes included in the predetermined operation mode. On the other hand, when the driver is allowed to select any one of the operation modes included in the predetermined operation mode, the target air-fuel ratio is set under a second constraint, under which the target air-fuel ratio tends to be set to a value richer than the target air-fuel ratio set under the first constraint. The internal combustion engine, the electric power/power reception/output device, and the electric motor are controlled so that an air-fuel ratio in the internal combustion engine becomes equal to the set target air-fuel ratio and a drive power corresponding to the set required drive power is output.
As in the hybrid vehicle to which the method is applied, when the driver is allowed to select any one of the operation modes included in the predetermined operation modes, it is assumed that fuel-supply cutoff in response to an instruction to decelerate the vehicle will be performed relatively frequently. In such a case, the NOx purification efficiency may be reduced, because a great amount of air is supplied to the exhaust gas purification catalyst due to fuel-supply cutoff and, therefore, oxygen adheres to the exhaust gas purification catalyst. To avoid such inconvenience, when the driver is allowed to select any one of the operation modes included in the predetermined operation mode, the target air-fuel ratio is set to a value richer than the target air-fuel ratio set when the driver is not allowed to select any one of the operation modes included in the predetermined operation mode. Thus, the amount of oxygen supplied to the purifying device can be reduced before fuel-supply is cut off or when fuel-supply is restarted after fuel-supply is cut off. As a result, even if a relatively great amount of air is supplied to the purifying device when fuel-supply is cut off, reduction in the NOx purification efficiency of the catalyst is suppressed, and, consequently, exhaust gas is purified more effectively.